Re: Issue: repack semi-frequently fails on Windows (msysgit) - suspecting file descriptor issues

hi,

i made a screenshot a couple of weeks ago (attached), and it seems to match 
your description.

rupert.


On Thursday, April 16, 2015 at 5:28:54 PM UTC+2, Jeff King wrote:

On Thu, Apr 16, 2015 at 01:35:05PM +0200, Andreas Mohr wrote: 

quoted

I strongly suspect that git's repacking implementation 
(probably unrelated to msysgit-specific deviations, 
IOW, git *core* handling) 
simply is buggy 
in that it may keep certain file descriptors open 
at least a certain time (depending on scope of implementation/objects!?) 
beyond having finished its operation (rename?). 

Hrm. I do not see anything in builtin/fetch.c that closes the packfile 
descriptors before running "gc --auto". So basically the sequence: 

  1. Fetch performs actual fetch. It needs to open packfiles to do 
     commit negotiation with other side (the hard work is done 
     by an index-pack subprocess, but it is likely we have to access 
     _some_ objects). 

  2. The packfiles remain open and mmap'd (at least on Linux) in the 
     sha1_file.c:packed_git list. 

  3. We spawn "gc --auto" and wait for it to finish. While we are 
     waiting, the descriptors are still open, but "gc --auto" will not be 
     able to delete any packs. 

But this seems too simple to be the problem, as it would mean that just 
about any "gc --auto" that triggers a full repack would be a problem (so 
anytime you have about 50 packs). But maybe the gc "autodetach" behavior 
means it works racily. 

I was able to set up the situation deterministically by running the 
script below: 

-- >8 -- 
#!/bin/sh 

# XXX tweak this setting as appropriate 
PATH_TO_GIT_BUILD=$HOME/compile/git 
PATH=$PATH_TO_GIT_BUILD/bin-wrappers:$PATH 
rm -rf parent child 

# make a parent/child where the child will have to access 
# a packfile to fulfill another fetch 
git init parent && 
git -C parent commit --allow-empty -m base && 
git clone parent child && 
git -C parent commit --allow-empty -m extra && 

# we want to make our base pack really big, because otherwise 
# git will open/mmap/close it. So we must exceed core.packedgitlimit 
cd child && 
$PATH_TO_GIT_BUILD/test-genrandom foo 5000000 >file && 
git add file && 
git commit -m large file && 
git repack -ad && 
git config core.packedGitLimit 1M && 

# now make some spare packs to bust the gc.autopacklimit 
for i in 1 2 3 4 5; do 
        git commit --allow-empty -m $i && 
        git repack -d 
done && 
git config gc.autoPackLimit 3 && 
git config gc.autoDetach false && 
GIT_TRACE=1 git fetch 

I also instrumented my (v1.9.5) git build like this: 

diff --git a/builtin/fetch.c b/builtin/fetch.c 
index 025bc3e..fc99e5e 100644 
--- a/builtin/fetch.c 
+++ b/builtin/fetch.c 
@@ -1174,6 +1174,12 @@ int cmd_fetch(int argc, const char **argv, const 
char *prefix) 
         list.strdup_strings = 1; 
         string_list_clear(&list, 0); 
  
+        { 
+                struct packed_git *p; 
+                for (p = packed_git; p; p = p->next) 
+                        trace_printf("pack %s has descriptor %d\n", 
+                                     p->pack_name, p->pack_fd); 
+        } 
         run_command_v_opt(argv_gc_auto, RUN_GIT_CMD); 
  
         return result; 
diff --git a/builtin/repack.c b/builtin/repack.c 
index bb2314c..e8b29cf 100644 
--- a/builtin/repack.c 
+++ b/builtin/repack.c 
@@ -105,6 +105,7 @@ static void remove_redundant_pack(const char 
*dir_name, const char *base_name) 
         for (i = 0; i < ARRAY_SIZE(exts); i++) { 
                 strbuf_setlen(&buf, plen); 
                 strbuf_addstr(&buf, exts[i]); 
+                trace_printf("unlinking %s\n", buf.buf); 
                 unlink(buf.buf); 
         } 
         strbuf_release(&buf); 

to confirm what was happening (because of course on Linux it is 
perfectly fine to delete the open file). If this does trigger the bug 
for you, though, it should be obvious even without the trace calls. :) 

-Peff 

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